In a "hot dip" coating process, a moving substrate such as steel is coated with a corrosion resistant material such as zinc by feeding the substrate through a coating bath. The substrate emerges from the bath along a generally upward, vertical feed path with molten coating material deposited on its surfaces.
The profile of the coating deposited on the substrate must be controlled to assure a substantially uniform coating on the resultant product. Profile control is also important to prevent wasteful deposition of excessively thick coatings and to assure that the coated substrate will perform in a consistent and desired manner in such handling processes as coiling, stacking and shipping, and in such fabrication processes as die forming and welding.
There are two aspects of profile control which require careful regulation. The first aspect is coating thickness which is expressed in the art as coating "weight". Coating thickness is specified in ounces per square foot, when a coated product is ordered. The second aspect is coating distribution which is often non-uniform in cross-section across the width of the substrate.
The crystalline structure of the coating material formed as the coating solidifies must also be controlled. A problem well known in the art is caused when crystal aggregates spread out from nuclei and join each other to form a more or less regular pattern resembling somewhat the appearance of a frosted window pane. This crystal formation is known as "spangle."
The prior art has disclosed several ways of controlling the thickness of the coating material and the spangle that occurs during solidification of the coating material.
U.S. Pat. No. 4,041,895 to Overton et al issued Aug. 16, 1977 and assigned to the same assignee of the present invention teaches the use of air knives to control the coating thickness. The air knives discharge a pressurized fluid such as air onto a moving substrate that emerges from a coating bath. The pressurized air screeds excess coating material from the substrate leaving a coating material deposit having a desired thickness and distribution.
One common process of cooling the coating material on the substrate taught by the prior art is by the use of a water spray impinged onto the coated substrate a distance downstream from the profile control device along the substrate feed path, which aids in solidifying the coating material. This cooling process improves the size of the spangles on the finished galvanized product. One approach made use of a water spray to cool the coating material in conjunction with the use of air knives to control the coating thickness and distribution. This was done by directing a water spray against an inclining surface of the air knife nozzle assembly so the water spray was deflected toward the coated substrate surface. This technique however has not proved to work in a satisfactory manner to obtain reliable, superior, surfaces having reduced spangle size.
An object of the present invention is to provide a new and improved method and apparatus for use in combination with an air knife thickness and distribution control device that will provide improved spangle control during the solidification of a coating material on a substrate.